Why does "singer" have /ŋ/ and "longer" have /ŋg/?

The word "singer" is pronounced as /ˈsɪŋ.ər/ having /ŋ/ and not /ŋg/ in Standard Englishes. On the other hand, the word "longer" is pronounced as /ˈlɒŋɡər/ having /ŋɡ/ and not just /ŋ/ in Standard Englishes. "Longer" is derived from "long" which only has /ŋ/.

There are some other words as well, like younger, youngest and the superlative form of "long": longest = /ŋg/

And those which have only /ŋ/: ringer, hanger, banger.

I read this answer (“English” pronounced as /ˈɪŋlɪʃ/ ING-LISH [closed]) and it suggests:

Word-internal [ŋg] mostly remained (as in finger, anger), but an originally word-final -ng- that becomes word-internal only as the result of the addition of certain suffixes is pronounced [ŋ] (e.g. singing, singer).

The -ng- in English is not word-final, and -lish is not a suffix here, so it isn't expected to be pronounced [ŋ] according to the pronunciation patterns of the most commonly described dialects.

The er is a suffix in both "longer" and "singer" but they are pronounced differently.

Why is "singer" not pronounced as /ˈsɪŋɡər/ or "longer" as /ˈlɒŋər/?

Is "longer" an exception to this rule or there is a specific reason?

Singer and longer both end in the letters -er, but they don't end in the same suffix: singer ends in the -er suffix that forms agent nouns, while longer ends in the -er suffix that forms comparative adjectives.

This is the reason why I included the word "certain" in "the addition of certain suffixes".

Words ending in the comparative suffix -er or the superlative suffix -est could be said to be an exception as a class to the general pattern of suffixes not altering the pronunciation of [ŋ].

But there are very few words with the relevant sequence -ng- + comparative -er, since there aren't many monosyllabic adjectives ending in [ŋ]. In fact, there are only three adjectives that have inflected forms with [ŋg]: younger/youngest, stronger/strongest, longer/longest (all common). The adjective wrong, also common, has what I think are uncommon inflected forms that are pronounced according to dictionaries and my own intuition with [ŋ]: wronger, wrongest. So it's arguable whether there is a regular exception for the category of comparative and superlative forms, or it's just a matter of these specific three adjectives having special comparative and superlative forms.

The same distribution of [ŋ] and [ŋg] in adjective forms is described (and prescribed) by John Walker in his Critical Pronouncing Dictionary (1803) so it seems to have been around in its usual modern form for a least a couple of centuries. But Walker also mentions variation in his time, saying that in Ireland comparative adjective forms like longer are "generally pronounced" with [ŋ] rather than [ŋg] (lxxxi).

In informal off-the-cuff formations, you can sometimes find comparatives or superlatives formed from words of two or more syllables ending in -ing, which I believe like wronger, wrongest would not be pronounced with [ŋg], but with [ŋ]: things like charmingest and boringer.

When you know that the agent noun "one that longs" (longer) is pronounced without a /ɡ/: /lɒŋ.ə/ (Longman Pronunciation Dictionary, RP), this might be explained cursorily by referring to the fickleness of usage .